Heat-shock Protein 90 as an Antimalarial Target

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CHAPTER 14

Heat-shock Protein 90 as an Antimalarial Target ANKIT K. ROCHANI, MEETALI SINGH AND UTPAL TATU* Department of Biochemistry, Indian Institute of Science, Bangalore-12, India *Email: [email protected]

14.1 History of Malaria Malaria is Latin for ‘‘bad air’’. It is an ancient protozoan infection that has afflicted humans throughout the history of mankind resulting in high morbidity and mortality. However, the causative agent for malaria was not known until the late nineteenth century when the parasite responsible for causing malaria was discovered by French army surgeon Charles Louis Alphonse Laveran, who won the Nobel Prize for Physiology or Medicine in 1907. Sir Ronald Ross in the late 1890s discovered the role of mosquitos in disease transmission, for which he was awarded the Nobel Prize in Medicine in 1902. Sir Ronald Ross observed for the first time the parasite oocysts in the midgut of the mosquito in 1897. In 1948, Henry Shortt and Cyril Garnham identified the pre-erythrocytic phase of the life cycle of the malaria parasite. This set of events made a major contribution towards the understanding of the transmission of one of the most lethal protozoan infections.1 3.3 billion people worldwide are at risk of contracting malaria. WHO reports 219 million cases of malarial in 2010 and over 600,000 deaths.2 Most infections and deaths, amounting to 91% of cases, happen in Africa. However, other regions including India, Latin America and some parts of the Middle East are RSC Drug Discovery Series No. 37 Inhibitors of Molecular Chaperones as Therapeutic Agents Edited by Timothy Machajewski and Zhenhai Gao r The Royal Society of Chemistry 2014 Published by the Royal Society of Chemistry, www.rsc.org

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also affected. The global economic growth due to international travel and interactions has led to a rampant increase in cases of malaria in non-endemic regions. Malaria poses a major risk to non-immune pregnant females, people with HIV/AIDS, young children and immigrants from endemic regions.2 Over 86% of deaths globally due to malaria happen in children. The Centre for Disease Control (CDC) has recommended prophylactic treatment for malaria in order to prevent cross-over cases of malaria due to traveling to malarial endemic regions.3 Malaria is caused by a protozoan parasite belonging to Plasmodium genus. Five species of Plasmodium can infect humans, namely P. falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi. The first two of these, i.e. Pf and Pv, account for the highest morbidity and mortality, whereas P. ovale and P. malariae generally cause a milder form of malaria. P. knowlesi generally infects monkeys but rarely can also cause infection in humans. Human malaria is transmitted by the female Anopheles mosquito.4 Tropical areas with low socio-economic demography are most affected due to lack of hygiene and plenty of breeding grounds for mosquitos.

14.2 Malaria and Prevention Malaria infection in humans is caused by sporozoite injection by feeding mosquitos. These sporozoites then, in turn, invade hepatocytes. In hepatocytes exoerythrocytic schizogony takes place, leading to the release of merozoites in the blood stream. These merozoites invade erythrocytes resulting in the ring stage of parasites. Active metabolism and proteolysis of hemoglobin is followed by development of trophozoites. Asexual division of trophozoites leads to formation of the schizont stage. Finally, merozoites are released by infected red blood cells (RBCs) and the cycle of erythrocyte invasion reinitiates. Most of the malaria symptoms are associated with the blood stage of the parasite. Fever episodes observed in malaria patients are due to synchronous lysis of infected erythrocytes.4,5 If untreated, disease prognosis could lead to complicated malaria symptoms and in the case of P. falciparum advanced disease could result in manifestation of cerebral malaria.6 There are three strategies of combating malaria: a) elimination of vector, b) antimalarial drugs and c) vaccination against malaria. Elimination of vector is considered to be the most cost-effective way of combating malarial infection. On the other hand development of antimalarials has been a major challenge for the healthcare sector. Rapid emergence of drug resistance has made it necessary to develop new antimalarial drugs. An effective antimalarial vaccine is still futuristic and in the developmental pipeline.4,7,8 There are mainly two ways to control mosquitoes: a) preventing the contact of mosquitos to humans and b) elimination of mosquitoes using insecticides. Mosquito repellents are most commonly used for prevention of diseases. But this prevention is not sufficient for controlling malaria. On the other hand, use of insecticides had provided minor help in controlling the spread of disease and has also cleared infection in some regions such as North America, Russia and

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some parts of Europe. DDT (dichlorodiphenyltrichloroethane), the first effective insecticide, was invented by Dr. Paul Muller, for which he was awarded the Nobel Prize in Physiology or Medicine in 1948. There are reports of this molecule being a possible carcinogen.9 DDT was one of the most used molecules in the antimalarial campaign by WHO (World Health Organization) before it was banned in the USA from 1972. It is commonly being used in India. It seems that the one of the most environmental friendly strategies to fight malaria is to find new drug targets and their respective targeting molecules. The journey from the identification of molecules from cinchona bark to the Artemisia annua plant suggests that the answer for combating the disease in the human body comes from Nature. Current antimalarial drug therapy acts at merozoites, at primary and secondary schizonts and on the intra-erythrocyte developmental stages of merozoites or gametocytes.4,10

14.3 Current Antimalarial Drugs and Drug Targets Malaria is an intracellular infection. The parasite resides in the RBCs of the host. An effective antimalarial drug has to pass through three cellular membranes (RBC, parasitophorus vacuole and parasite membrane) to have an antimalarial effect. Due to the complex biochemical properties of Plasmodium it has become difficult to make antimalarial drugs. The advent of ‘‘Omics’’ technologies like genomics, proteomics and metabolomics has relatively speeded up the methods of identification of novel drug targets. This in turn has provided momentum towards identification of active and potent antimalarial therapeutics. The broad classification of available antimalarial scaffolds and their respective molecular drug targets is described in Table 14.1. The structures of the antimalarial drugs in clinical use are shown in Figure 14.1.

14.4 Heat-shock Protein 90 as Antimalarial Drug Target Heat-shock protein 90 is a well-studied protein. It was first discovered as a protein up-regulated in response to heat shock. Decades of studies have now placed Hsp90 in the center of hub regulating key cellular processes like stress response, signal transduction, immunity, protein homeostasis, accumulation of drug resistance and many other processes.11 Heat-shock protein is one of the most abundant proteins and belongs to the GHKL family of ATPases having three distinctive domains: N-terminal domain harboring ATP binding site joined to the middle domain by a charged linker region of variable length. The middle domain harbors the catalytic arginine residue and a C-terminal domain is responsible for dimer formation. All these domains have specific sites for binding of co-chaperones and clients.12 The chaperoning function of this protein depends on its ATPase activity. As indicated in Figure 14.2 the unfolded protein interacts with ADP bound Hsp90

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Table 14.1

List of antimalarial scaffolds with their respective derivatives and their corresponding drug targets.

S. No. Scaffolds

Drug Targets

Derivatives

1

Targets ferriprotoporphyrin IX

Quinine Quinidine Chloroquine Hydroxychloroquine Amodiaquine Mefloquine Primaquine Sulfadoxine Proguanil Pyrimethamine Atovaquone

Quinoline 8-Aminoquinoline

2 3 4 5

Sulfonamides Guanidine Pyrmidine Quinolones

6

Polycyclic compounds

PABA competitive inhibitor Dihydrofolate reductase inhibitor Dihydrofolate reductase inhibitor Selective inhibitor of electron transport system of Plasmodium Binding to 30s ribosomal subunit and inhibits protein synthesis Unknown DNA intercalating agent, succinic acid dehydrogenase binding agent, mitochondrial electron transport chain inhibitor, cholinesterase inhibitor Molecule reacts with iron of heme of infected erythrocyte, which forms reactive free radical species, which is postulated to be lethal for parasite DOXP reductoisomerase inhibitor

Doxycycline Halofantrine Quinacrine

Artemisinin Dihydroartimisinin Artemether Artemotil Artesunate Fosmidomycin

complex. ATP binding triggers conformational changes and subsequent ATP hydrolysis leads to release of mature or folded client protein. Hsp90 inhibitors like GA and its analogues competitively bind to the ATP binding site of the N-terminal domain of Hsp90 in turn compromising the folding of the client proteins. Unfolded or misfolded proteins are thus subjected to the proteasomal degradation pathway. This phenomenon ultimately leads to cell death.11 Plasmodium falciparum has a complex digenetic life cycle, where it has to transmit from its definitive host mosquito to its human host. During such a transition the parasite experiences a heat shock of 10 1C. During the asexual life cycle of Plasmodium in human erythrocytes, release of merozoites from infected RBCs is accompanied by high febrile episodes. The host immune system puts forward its best defense strategies to the incoming parasite, yet Plasmodium successfully manifests the disease. Studies have shown that Plasmodium actually exploits these febrile episodes to its benefit. If parasites are subjected to a heat shock, then re-exposure to such a heat shock promotes stage transition from ring stage to trophozoite stage robustly during a second heat-shock episode.13 In the same study, the authors have shown the involvement of a classical chaperone, heat-shock protein 90


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Figure 14.1

Structures of Anti-malarial Drugs.

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Figure 14.2

Chapter 14

Mechanism of PfHsp90 Inhibitors.

(Hsp90), to be involved in a robust stage switch. Treatment of cells exposed to first heat shock with a specific Hsp90 inhibitor geldanamycin (GA) in turn interferes with parasite stage switch, thereby establishing the key role Hsp90 plays in survival during stress and exploiting these conditions for improved and faster manifestation of the infectious cycle by Plasmodium. Hsp90 functions in association with its partner proteins known as cochaperones, which modulate not only its ATPase activity but also prime Hsp90 to bind various different sets of client proteins. Pavithra et al. have shown inhibition of Hsp90 by GA in Plasmodium falciparum disrupts its complex with PfHsp70 and other co-chaperones, highlighting the importance of this multichaperone complex in a cyto-protective role.13 Hsp90 function is essential for Plasmodium survival. Reduction in parasitemia was observed upon GA treatment with an LD50 of 0.2 mM. Studies have also shown that GA treatment abrogates stage transition from ring stage to trophozoites, leading to persistence of rings.14 At the preclinical level studies have shown the efficacy of GA derivative 17AAG in mice models of malaria. Upon 17-AAG treatment (50 mg/kg b.w.) for P. berghei infection, mice survival was found to increase by up to two-fold in comparison to untreated mice.15 Studies on another rodent malaria model of P. yoelli also support this observation.16 PfHsp90 shares B70% identity with its human counterpart. One of the unique aspects of PfHsp90 is the presence of a long charged acidic linker region, which is 33 amino acids longer than the yeast linker region.17,18 PfHsp90 nucleotide binding site shows significant conservation with respect to yeast or

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human Hsp90 but there are subtle differences in the binding pocket. Differences in the side change rotamer conformation of Met84 result in an altered ceiling shape in the case of PfHsp90. Other amino acid substitution in PfHsp90, like that of V173I and of S38A, results in a constricted and hydrophobic posterior end of the binding pocket.17,18 It is reported that PfHsp90 has a high affinity towards GA binding and is a hyperactive ATPase compared to other Hsp90s.15 Also, yeast strain complemented with PfHsp90 is found to be more sensitive to Hsp90 inhibitionmediated growth arrest compared to yeast strain harboring wild-type Hsp90.19 These studies altogether provide proof of principle for the role of PfHsp90 as a potential antimalarial drug target. Drug resistance is a major problem in disease treatment. Studies from various groups have shown the role of Hsp90 in the emergence of drug resistance. Hsp90 is expressed at very high levels in cells acting as a specialized reservoir for folding of metastable client proteins. Hsp90, thus, can potentiate folding of genetic variants leading to the emergence of newer phenotypes. Experimental evidence has shown that Hsp90 potentiates drug resistance in fungal species.20–22 Therefore, the use of Hsp90 inhibitors can be exploited to overcome drug resistance and prospects of combination therapy in multi-drug resistant strains can be envisaged. The Hsp90 inhibitor GA is capable of arresting growth in a chloroquine-resistant strain of Plasmodium.23 Synergistic action of GA and chloroquine has been observed in inhibiting Plasmodium growth.23 Pallavi et al. have shown synergistic action of Trichostatin-A (histone deacetylase inhibitor) and GA in growth inhibition of Plasmodium, which is suggestive of the potential of combinatorial therapy with Hsp90 inhibitors and available drugs in the market.15 The approach of targeting PfHsp90 protein as a drug target has been one of the recent developments. The three major approaches adopted to find novel drug molecules targeting Plasmodium Hsp90 protein are a) repurposing strategy, b) high-throughput screening of molecules and c) rational approach to new drug discovery.

14.5 Hsp90 Targeted New Antimalarial Drug Discovery The drug development process typically takes 10–15 years. Repurposing strategy is often used to speed up this process and has uncovered some of the blockbuster drugs. For example, the alternate use of Sidenafil citrate in erectile dysfunction became a drug called Viagra by Pfizer. Thalidomide was initially introduced as an anti-nausea and sedative drug in the 1950s, but it was withdrawn from the market in the 1960s during the Phase IV trial due to reports of birth defects in 10,000 children from nearly 46 countries. The same molecule was re-introduced for the treatment of erythema nodosum leprosum and was approved by the USFDA in 1998.24 On similar lines, the use of Hsp90 inhibitors has recently been diversified from cancer to find a novel treatment solution for infections like malaria, surra, sleeping sickness, candidiasis, HIV, influenza, HBV and others.25

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14.5.1

Chapter 14

Repurposing for PfHsp90 Inhibitors

Using the repurposing strategy GA (geldanamycin) was identified as the first molecular lead targeting PfHsp90 protein in 2000.14,25 Further in 2010 it was reported that 17-AAG (17-allyl amino 17-demethoxy geldanamycin), a semisynthetic derivative of GA, showed high binding specificity to PfHsp90.15 17AAG was the first human Hsp90 (hHsp90) inhibitor that entered Phase III clinical trials for treatment of cancer. By using the repurposing strategy it was observed that the binding affinity of 17-AAG towards PfHsp90 is greater than that of hHsp90.15 This result was later confirmed by the treatment of malaria infection in mice model at 50 mg/kg b.w. with 40–50% survival. When this chapter was written there were 15 novel hHsp90 inhibitors in clinical trials for treatment of cancer from approximately 35 public and private ventures. Their use as antimalarial molecules by targeting PfHsp90 protein is still an unexplored area of research.

14.5.1.1

Ansamycins Antimalarial Antibiotic

GA and its analogue 17-AAG belong to a class of macrocyclic ansamycins antibiotics as shown in Figure 14.3a. Structurally they are benzoquinone ansamycin derivatives. They are naturally occurring polyketide compounds. Rifampicin was the first ansamycin antibiotic introduced as a drug for treatment of tuberculosis, leprosy and AIDS-associated mycobacterium infection. GA is a fermentation product of a soil organism called Streptomyces hygroscopicus var geldanus. It was isolated in 1969 from fermentation broth.26 3-Amino 5-hydroxy benzoic acid (AHBA) is the building block of the antibiotic. It has been used extensively for carrying out structure activity relationships (SARs) and fragment-based drug discovery for binding toward hHsp90.25,27,28 Clinically, GA has been reported to have hepatoxicity, which makes it unsuitable for human consumption even though it shows considerable therapeutic effect. Hence, 17-AAG was the first Hsp90 inhibitor to enter human trials due to relative safety. As indicated previously these molecules get positioned in the ATP binding pocket of PfHsp90 protein as shown in Figure 14.3b. The figure also shows comparatively the binding site for ATP and 17-AAG. Also, these molecules are reported to be negative toward mutagenicity studies. Also, from the clinical trial data of 17-AAG it is clear that the molecule has never been shown to have any life-threatening toxicity.29 The most common drug related toxicities were nausea, vomiting, fatigue, pain and rise in liver transaminases like ALT & AST (alanine and aspartate transaminases) under high dosing regimen.30–34 This provides an added advantage for Hsp90 targeted inhibitors over other classical anticancer drugs for repurposing them for the treatment of malaria.

14.5.1.2

Other Naturally Occurring Antimalarial Drugs

The naturally occurring plant Azadirachta indica (Indian neem tree) is known for its potent antimalarial activity. A tetranortriterpenoid called gedunin35

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Figure 14.3a

Geldanamycin and 17-allylamino 17-demethoxy geldanamycin.

Figure 14.3b

Molecular docking of ansamycin scaffold to N-terminal domain of PfHsp90 protein.

isolated from Azadirachta indica is reported to have affinity towards hHsp90. The molecules have been reported to have antimalarial activity.35 Hence it can be hypothesized that the molecule may exhibit affinity towards PfHsp90. This hypothesis remains to be validated by experimental evidence. Flavonoids were the first phytochemicals to be introduced as hHsp90 binding agents. However, none of these phytochemicals or their derivatives have been introduced for clinical applications due to their non-specificity. The idea of exploring these phytochemicals for their PfHsp90 inhibition remains a topic of discussion.

14.5.2

High-throughput Screening of PfHsp90 Inhibitors

High-throughput screening (HTS) of a molecular library is one of the most common and reliable strategies for carrying out new drug discovery for an

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Table 14.2

List of chemical databases and libraries along with their URL (Uniform Resource Locator) addresses that provide updates on newly introduced information on PfHsp90 inhibitors.

S. No. Source

URL

Type

1 2 3

http://pubchem.ncbi.nlm.nih.gov/ http://www.chemspider.com/ https://www.discoverygate.com/dg3/ DiscoveryGate/DiscoveryGate.jsp http://zinc.docking.org/ http://www.vitasmlab.com/downloads http://www.sigmaaldrich.com/chemistry/ drug-discovery/validation-libraries.html http://www.prestwickchemical.com/ index.php?pa ¼ 3 http://www.nihclinicalcollection.com/ http://www.chembridge.com/ screening_libraries/fragment_library/ http://www.asinex.com/downloadzone.html http://www.enamine.net/ http://www.maybridge.com/portal/ alias__Rainbow/lang__en/tabID__138/ DesktopDefault.aspx http://www.cerep.fr/cerep/Users/pages/ ProductsServices/pharmacoetADME.asp

Database Database Database

8 9

PubChem Chemspider Discovery Gate Zinc Vitasmlab Sigma Lopac Prestwick Chemical NIH Chembridge

10

Asinex

11 12

Enamine Maybridge

13

Cerep

4 5 6 7

Database Database and library Database and library Library Database and library Database and library Database and library Database and library Database and library Database and library

identified novel drug target. There are nearly 5394 potential drug targets for Plasmodium species.36 PfHsp90 (PDB ID: 3IED and 3k60) became one of the most commonly studied drug targets for carrying out HTS for identification of new drug-like molecules. Benzoquinone ansamycin is the only well-characterized molecular scaffold that showed antimalarial activity and whose target is known to be PfHsp90 protein. HTS by some groups has shed light on some gray areas of molecular chemistry, which has provided around 1899 new molecular structures that have been found to have binding affinity towards PfHsp90 protein.37 There are various databases available that are normally used for HTS of molecules targeting a variety of drug targets and these are listed in Table 14.2. All these databases and libraries can be used for carrying out necessary searches related to ligands binding PfHsp90 protein and other chemical properties.

14.5.3

Rational Approach to Hsp90 Targeted Drug Discovery

Understanding of chemistry of a molecular scaffold or drug-like molecule is the first step towards a rational approach to drug discovery. For a new drug target; it becomes difficult to design a derivative due to limited knowledge about the available molecular scaffolds. It is also a known fact that all molecules that bind to a drug target may not be a potential drug candidate. The Lipinski rule of five

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provides some theoretical understanding of molecular structure but its application is limited due to the dynamic nature of therapeutic effects. GA derivatives bind to PfHsp90 at the ATP binding site of the target. There are many hHsp90 derivatives that are derived from purine and ansamycin scaffolds that compete with ATP for binding to Hsp90.38 Few of these molecules show considerably good binding affinity towards the protein but whether these molecules prove to be better antimalarial drugs remains an open question.

14.6 Future of PfHsp90 Inhibitors as Antimalarials Human Hsp90 inhibitors are now being examined under advanced clinical trials. The most studied hHsp90 inhibitors are 17-AAG, AUY922, IPI504 and STA9090.25 The idea of using Hsp90 from malaria as a drug target has grown significantly since its inception in 2000.39 The structure–activity relationship for scaffolds that can bind selectively to the N-terminal domain of PfHsp90 is in its nascence. From the knowledge at hand it is very clear that PfHsp90 proves to be a very valuable drug target towards the next generation of antimalarial therapy. This idea has also served as the first proof of principle that not only drugs but also the drug targets can be repurposed or researched for their alternative therapeutic implications. Hence, cancer is not the only condition in which Hsp90 can be used as drug target.39 Hsp90 has been proven as a new drug target for a variety of diseases from scientific findings. The summary of the last three decades suggests that we have a) a next-generation antimalarial drug target and its potential inhibitor b) around 15 highly potent hHsp90 inhibitors in advanced stages of clinical trials, which can be explored for their efficacy as antimalarial drugs c) use of Hsp90 as a drug target from various infectious micro-organisms, which can be explored for finding novel treatment regimens. 17-AAG was the first reported antimalarial molecule and PfHsp90 inhibitor in 2010 under both in vitro and in vivo conditions. Formulation of this molecule has been a major issue due to its highly hydrophobic characteristic. 17-Dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) is a water-soluble molecule and shares the molecular ring structure with 17-AAG. When this chapter was written this molecule was under Phase I clinical trial examination for cancer treatment. Substitution of 17-allyl amine side-chain by 17-demethoxyaminoethylamino created a molecule that is water soluble and closer to having drug-like characteristics. But its use as an antimalarial candidate remains unexplored.

References 1. M. H. Azizi and M. Bahadori, Arch. Iran. Med., 2013, 16, 131–135. 2. http://www.who.int/features/factfiles/malaria/en/.

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3. http://www.cdc.gov/malaria/malaria_worldwide/impact.html. 4. J. H. Block, in Organic Medicinal and Pharmaceutical Chemistry, J. M. Beale and J. H. Block, Lippincott Williams and Wilkins, USA, 2004, pp. 282–298. 5. T. N. Wells, P. L. Alonso and W. E. Gutteridge, Nat. Rev. Drug Discov., 2009, 8, 879–891. 6. N. Kheliouen, F. Viwami, F. Lalya, N. Tuikue-Ndam, E. C. Moukoko, C. Rogier, P. Deloron and A. Aubouy, Malar. J., 2010, 9, 220. 7. L. Schwartz, G. V. Brown, B. Genton and V. S. Moorthy, Malar J., 2012, 11, 11. 8. M. A. Thera and C. V. Plowe, Annu. Rev. Med., 2012, 63, 345–357. 9. http://www.atsdr.cdc.gov/toxprofiles/tp.asp?id ¼ 81&tid ¼ 20. 10. T. L. Lemke, in Foye’s Principles of Medicinal Chemistry, ed. T. L. Lemke and D. A. Williams, Lippincott Williams & Wilkins, USA, 2008, pp. 1084– 1111. 11. M. Taipale, D. F. Jarosz and S. Lindquist, Nat. Rev. Mol. Cell Biol., 2010, 11, 515–528. 12. L. H. Pearl and C. Prodromou, Annu. Rev. Biochem., 2006, 75, 271–294. 13. S. R. Pavithra, G. Banumathy, O. Joy, V. Singh and U. Tatu, J. Biol. Chem., 2004, 279, 46692–46699. 14. G. Banumathy, V. Singh, S. R. Pavithra and U. Tatu, J. Biol. Chem., 2003, 278, 18336–18345. 15. R. Pallavi, N. Roy, R. K. Nageshan, P. Talukdar, S. R. Pavithra, R. Reddy, S. Venketesh, R. Kumar, A. K. Gupta, R. K. Singh, S. C. Yadav and U. Tatu, J. Biol. Chem., 2010, 285, 37964–37975. 16. R. Mout, Z. D. Xu, A. K. Wolf, V. Jo Davisson and G. K. Jarori, Malar. J., 2012, 11, 54. 17. R. Kumar, S. R. Pavithra and U. Tatu, J. Biosci., 2007, 32, 531–536. 18. K. D. Corbett and J. M. Berger, Proteins, 2010, 78, 2738–2744. 19. D. Wider, M. P. Peli-Gulli, P. A. Briand, U. Tatu and D. Picard, Mol. Biochem. Parasitol., 2009, 164, 147–152. 20. S. L. Rutherford and S. Lindquist, Nature, 1998, 396, 336–342. 21. D. F. Jarosz and S. Lindquist, Science, 2010, 330, 1820–1824. 22. L. E. Cowen, S. D. Singh, J. R. Kohler, C. Collins, A. K. Zaas, W. A. Schell, H. Aziz, E. Mylonakis, J. R. Perfect, L. Whitesell and S. Lindquist, Proc. Natl Acad. Sci. USA, 2009, 106, 2818–2823. 23. R. Kumar, A. Musiyenko and S. Barik, Mol. Biochem. Parasitol., 2005, 141, 29–37. 24. http://www.nytimes.com/1998/07/17/us/thalidomide-approved-to-treatleprosy-with-other-uses-seen.html. 25. A. K. Rochani, M. Singh and U. Tatu, Curr. Pharm. Des., 2013, 19, 377–386. 26. C. DeBoer, P. A. Meulman, R. J. Wnuk and D. H. Peterson, J. Antibiot. (Tokyo), 1970, 23, 442–447. 27. K. Lee, J. S. Ryu, Y. Jin, W. Kim, N. Kaur, S. J. Chung, Y. J. Jeon, J. T. Park, J. S. Bang, H. S. Lee, T. Y. Kim, J. J. Lee and Y. S. Hong, Org. Biomol. Chem., 2008, 6, 340–348.

View Online



391

28. S. Eichner, H. G. Floss, F. Sasse and A. Kirschning, Chembiochem., 2009, 10, 1801–1805. 29. K. Jhaveri, T. Taldone, S. Modi and G. Chiosis, Biochim. Biophys. Acta, 2012, 742–755. 30. R. Bagatell, L. Gore, M. J. Egorin, R. Ho, G. Heller, N. Boucher, E. G. Zuhowski, J. A. Whitlock, S. P. Hunger, A. Narendran, H. M. Katzenstein, R. J. Arceci, J. Boklan, C. E. Herzog, L. Whitesell, S. P. Ivy and T. M. Trippett, Clin. Cancer Res., 2007, 13, 1783–1788. 31. S. Modi, A. Stopeck, H. Linden, D. Solit, S. Chandarlapaty, N. Rosen, G. D’Andrea, M. Dickler, M. E. Moynahan, S. Sugarman, W. Ma, S. Patil, L. Norton, A. L. Hannah and C. Hudis, Clin. Cancer Res., 2011, 17, 5132– 5139. 32. S. Pacey, M. Gore, D. Chao, U. Banerji, J. Larkin, S. Sarker, K. Owen, Y. Asad, F. Raynaud, M. Walton, I. Judson, P. Workman and T. Eisen, Invest. New Drugs, 2010, 30, 341–349. 33. E. A. Ronnen, G. V. Kondagunta, N. Ishill, S. M. Sweeney, J. K. Deluca, L. Schwartz, J. Bacik and R. J. Motzer, Invest. New Drugs, 2006, 24, 543–546. 34. E. I. Heath, M. Gaskins, H. C. Pitot, R. Pili, W. Tan, R. Marschke, G. Liu, D. Hillman, F. Sarkar, S. Sheng, C. Erlichman and P. Ivy, Clin. Prostate Cancer, 2005, 4, 138–141. 35. G. E. Brandt, M. D. Schmidt, T. E. Prisinzano and B. S. Blagg, J. Med. Chem., 2008, 51, 6495–6502. 36. http://tdrtargets.org. 37. Y. Wang, J. Xiao, T. O. Suzek, J. Zhang, J. Wang, Z. Zhou, L. Han, K. Karapetyan, S. Dracheva, B. A. Shoemaker, E. Bolton, A. Gindulyte and S. H. Bryant, Nucleic Acids Res., 2011, 40, D400–412. 38. E. B. Garon, R. S. Finn, H. Hamidi, J. Dering, S. Pitts, N. Kamranpour, A. J. Desai, W. Hosmer, S. Ide, E. Avsar, M. Rugaard Jensen, C. Quadt, M. Liu, S. M. Dubinett and D. J. Slamon, Mol. Cancer Ther., 2013, 12(6), 890–900. 39. E. Dolgin and A. Motluk, Nat. Med., 2011, 17, 646–649.